Explanation/Reference:
A: Question: I see errors and warnings during upgrade about connectivity or corruption Clean up orphaned sites, lists, and other database corruptions before you try upgrade again.
In the upgrade log files, you may see errors such as the following:
* WARNING The orphaned sites could cause upgrade failures.
* ERROR Database [Content Database Name] contains a site (Id = [Site Collection Identifier], Url = [Site Collection URL]) that is not found in the site map.
Fix any orphaned items or database corruptions, and then run upgrade again.
B: Question: The log says I have missing templates, features, or other server-side customizations Identify all server-side customizations and install them before you upgrade You can use the test-spcontentdatabase Windows PowerShell cmdlet before you upgrade the database to identify missing files.
In the upgrade log files, you may see errors such as the following:
* ERROR Found a missing feature Id = [Feature Identifier]
D: Delete the duplicate content type.
Reference: Troubleshoot database upgrade issues in SharePoint 2013
https://technet.microsoft.com/en-us/library/cc262967.aspx